The broad, long term objective of this research is to investigate the mechanism by which mammary epithelial cells initiate and maintain G0 growth arrest. The specific aims will investigate the regulation and function of C/EBPdelta, a transcription factor that is highly induced in mammary epithelial cells during G0 growth arrest and prior to the onset of apoptosis. The hypothesis is that C/EBPdelta regulates mammary epithelial cell G0 growth arrest. Loss of C/EBPdelta alters growth control and predisposes to mammary tumorigenesis. The specific aims are: 1) Investigate the transcriptional regulation of C/EBPdelta gene expression in mammary epithelial cells, 2) Investigate the intracellular signal transduction pathway that activates C/EBPdelta gene expression in mammary epithelial cells, 3) Investigate the posttranscriptional regulation of C/EBPdelta in mammary epithelial cells, 4) Investigate the role of C/EBPdelta in mammary gland development, programmed cell death and tumorigenesis in vivo in C/EBPdelta knockout mice. The health-relatedness of this project is that it fills gaps in our knowledge about G0 growth regulation in mammary epithelial cells, the principal cell population implicated in human breast cancer. Few G0 genes have been identified and little is known about their regulation. The experimental design will utilize a well-characterized mammary epithelial cell line (HC11) to investigate the role of extracellular ligands, intracellular signal transduction pathways and transcriptional and posttranscriptional control mechanisms in C/EBPdelta regulation. C/EBPdelta knockout mice will be used to address the physiological relevance of C/EBPdelta in mammary gland biology in vivo. The methodology will use transfection of C/EBPdelta promoter-reporter and indicator transcript constructs to investigate transcriptional and posttranscriptional regulation. Specific DNA-protein interactions are assessed by band shift and antibody interference assays. Gene expression is evaluated by northern and western blot. In vivo analysis of C/EBPdelta regulation and function will use tissue morphology, in situ labelling, tumor detection and characterization methods. The results will provide a better understanding of G0 growth arrest in mammary epithelial cells and new insights into the etiology, progression and possibly treatment of breast cancer.